The interband cascade laser (ICL) is a promising semiconductor coherent source for the mid-IR (defined here as 2.5-6 μm) spectral region and, potentially, longer wavelengths.
The basic concept of the ICL was invented by Rui Yang in 1994, while he was at the University of Toronto, and was patented shortly later while he was at the University of Houston. See U.S. Pat. No. 5,588,015, “Light Emitting Devices Based on Interband Transitions in Type II Quantum Well Heterostructures.”
The ICL differs from the mid-IR quantum cascade laser (QCL) in that ICLs employ interband transitions rather than the intersubband active transitions used by QCLs. The carrier lifetime associated with interband transitions is typically three orders of magnitude longer than for intersubband transitions, which ultimately results in more than an order-of-magnitude lower drive power in the ICL. The active transitions in most ICLs are spatially indirect (type-II), with electron and hole wavefunctions peaking in adjacent electron (e.g., InAs) and hole (e.g., GaInSb) quantum wells (QWs), respectively. In contrast to conventional diode lasers, both ICLs and QCLs have two n-type contact regions, at both ends of the device, for current flow.
Each active stage of an ICL contains a short-period chirped superlattice that displays highly-anisotropic electrical conduction. The active stages of QCLs are also electrically anisotropic, although QCLs typically have bulk InP cladding layers that are electrically isotropic.
In addition, to provide optical confinement, most ICLs employ moderately n-doped InAs/AlSb superlattice cladding layers, which also have anisotropic electrical resistance because the electron mobility in the plane of the short-period superlattice is far higher than that along the growth axis.
Following the initial development of the ICL, critical improvements to the basic ICL structure, such as including more than one hole well to form a hole injector, were made in a joint patent by Dr. Yang in collaboration with two of the inventors of the present invention. See U.S. Pat. No. 5,799,026 to Meyer et al., “Interband Quantum Well Cascade Laser with a Blocking Quantum Well for Improved Quantum Efficiency.” This was followed by a number of additional patents by some of the inventors of the present invention, which introduced numerous further improvements to the ICL structure and operation. See U.S. Pat. No. 8,125,706 to Vurgaftman et al., “High Temperature Interband Cascade Lasers”; U.S. Pat. No. 8,493,654 to Vurgaftman et al., “High Temperature Interband Cascade Lasers”; U.S. Pat. No. 8,290,011 to Vurgaftman et al., “Interband Cascade Lasers”; U.S. Pat. No. 8,385,378 to Vurgaftman et al., “Interband Cascade Lasers”; U.S. Pat. No. 8,798,111 to Vurgaftman et al., “Interband Cascade Lasers with Engineered Carrier Densities”; and U.S. Pat. No. 9,059,570 to Vurgaftman et al., “Interband Cascade Lasers with Engineered Carrier Densities.”
One of the most critical improvements was to substantially increase the doping density in the electron injector, so as to increase the electron density in the active QWs and thereby lower the lasing threshold current density (“carrier rebalancing”). U.S. Pat. No. 8,798,111 and U.S. Pat. No. 9,059,570, supra. This dramatically reduced the threshold power for ICL operation, to as low as 29 mW at room temperature.
However, most of the measures taken thus far to optimize the configurations for electrical contact and the waveguide for the lasing mode have followed conventional general principles known to the semiconductor laser community.